Isothermal amplification under low salt condition

ABSTRACT

Provided herein are methods and kits for isothermal nucleic acid amplifications that use a target nucleic acid template; a reaction mixture comprising a DNA polymerase having a strand displacement activity, a deoxyribonucleoside triphosphate (dNTP) mixture, a primer with a 3′ end and a 5′ end, a molecular crowding reagent, and a buffer solution for amplifying the target nucleic acid template. The buffer solution maintains a low salt concentration of the reaction mixture, and wherein the salt concentration results in a melting temperature (T m ) of the primer at least 10° C. below the reaction temperature. The amplification is effected under isothermal condition.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.14/225,887, filed on Mar. 26, 2014, entitled ISOTHERMAL AMPLIFICATIONUNDER LOW SALT CONDITION, which is hereby incorporated by reference inits entirety.

FIELD OF INVENTION

The invention generally relates to methods and kits for performingisothermal amplification reactions employing molecular crowding reagentsunder low salt conditions.

BACKGROUND

DNA amplification is a process of replicating a target double-strandedDNA (dsDNA) to generate multiple copies. Since individual strands of adsDNA are antiparallel and complementary, each strand may serve as atemplate strand for the production of its complementary strand. Thetemplate strand is preserved as a whole or as a truncated portion andthe complementary strand is assembled from deoxyribonucleosidetriphosphates (dNTPs) by a DNA polymerase. The complementary strandsynthesis proceeds in the 5′→3′ direction starting from the 3′ terminalend of a primer sequence that is hybridized to the template strand. Avariety of efficient nucleic acid amplification techniques are currentlyavailable such as polymerase chain reaction (PCR), ligase chain reaction(LCR), self-sustained sequence replication (3SR), nucleic acid sequencebased amplification (NASBA), strand displacement amplification (SDA),multiple displacement amplification (MDA), or rolling circleamplification (RCA). Many of these techniques generate a large number ofamplified products in a short span of time.

Whole-genome amplification (WGA) involves non-specific amplification ofa target DNA. WGA is often achieved by MDA employing randomoligonucleotide primers (e.g., NNNNN*N) for priming DNA synthesis atmultiple locations of the target DNA along with a high-fidelity DNApolymerase having a strand displacing activity (e.g., Phi29 polymerase).Even though currently available commercial WGA systems such as GenomiPhi(GE Healthcare, USA) and RepliG (Qiagen) kits provide optimal resultswith an input DNA of 1 nanogram or more, performance of these systems ispoor when the target DNA is available only in smaller quantities or whenamplification of DNA from a few or single cells is performed.

Despite these advancements, there remains a need for developing moreefficient whole-genome nucleic acid amplification methods that havelower bias in terms of sequence coverage and produce lower levels ofnon-specific, background amplification. Amplification of trace amountsof target DNA using conventional methods often results in incompleteamplification of DNA sequences leaving “dropouts” in sequence coverageand amplification bias wherein DNA sequences are amplified unevenly.Further, products of the amplification reaction (amplicons) may oftenanneal among themselves leading to the generation of undesirablechimeric products. Efficient methods for non-specifically amplifyingtrace amounts of target DNA are therefore highly desirable.

BRIEF DESCRIPTION

In some embodiments, nucleic acid amplification methods are providedthat utilize a molecular crowding reagent under low salt condition and aprimer with low melting temperature for amplifying a target nucleic acidto generate amplicons.

In some embodiments, an isothermal amplification method for producing atleast one amplicon based on a target DNA is provided. The methodcomprises the steps of providing a target nucleic acid template andcontacting the target nucleic acid template with a reaction mixturecomprising a DNA polymerase having a strand displacement activity, adeoxyribonucleoside triphosphate (dNTP) mixture, a primer with a 3′ endand a 5′ end, a molecular crowding reagent, and a buffer solution,wherein the buffer solution maintains a salt concentration of thereaction mixture between 10 to 30 mM. The amplification is effectedunder isothermal condition at a constant reaction temperature, whereinthe salt concentration optimizes a melting temperature (T_(m)) of theprimer at least 10° C. below the reaction temperature.

In some embodiments, an isothermal amplification method for producing atleast one amplicon based on a target DNA is provided. The methodcomprises the steps of, providing a target nucleic acid template;contacting the target nucleic acid template with a reaction mixturecomprising a DNA polymerase having a strand displacement activity, adeoxyribonucleoside triphosphate (dNTP) mixture, a primer with a 3′ endand a 5′ end, polyethylene glycol as a molecular crowding reagent, and abuffer solution, wherein the buffer solution maintains a saltconcentration of the reaction mixture at 15 mM. The amplification iseffected under isothermal condition at a constant reaction temperatureof 30° C., wherein the salt concentration optimizes a meltingtemperature (T_(m)) of the primer at least 10° C. below the reactiontemperature.

In some embodiments, kits for isothermal DNA amplification are provided.The kits comprise a DNA polymerase having strand displacement activity,a molecular crowding reagent; and a buffer that provides a final saltconcentration between 10 mM to 20 mM during amplification.

DRAWINGS

These and other features, aspects and advantages of the invention willbecome better understood when the following detailed description is readwith reference to the accompanying figures.

FIG. 1A illustrates the kinetics of an MDA reaction in absence of lowsalt and molecular crowding reagents.

FIG. 1B illustrates the kinetics of an MDA reaction without molecularcrowding reagents, in presence of low salt.

FIG. 1C illustrates the effectiveness of a molecular crowding reagentand low salt condition to increase the kinetics and sensitivity of anMDA reaction from a single cell.

FIG. 2 illustrates the effectiveness of a molecular crowding reagent andlow salt condition in reducing background non-specific amplification,allowing for a greater percentage of the total reads to be mapped to thetarget genome.

FIG. 3 illustrates the effectiveness of a molecular crowding reagent andlow salt condition to increase the overall genome sequence coverage atvarying depths and the amplification balance of DNA amplificationreactions initiated from a single cell.

DETAILED DESCRIPTION

To more clearly and concisely describe and point out the subject matterof the claimed invention, the following definitions are provided forspecific terms that are used in the following description and appendedclaims.

As used herein, the term “target DNA” refers to a DNA sequence of eithernatural or synthetic origin that is desired to be amplified in a DNAamplification reaction. The target DNA acts as a template in a DNAamplification reaction. Either a portion of a target DNA or the entireregion of a target DNA may be amplified by a DNA polymerase in a DNAamplification reaction to produce amplification products or amplicons.Amplicons may include multiple copies of the target DNA or multiplecopies of sequences that are complementary to the target DNA. The targetDNA may be obtained from a biological sample in vivo or in vitro. Forexample, the target DNA may be obtained from a bodily fluid (e.g.,blood, blood plasma, serum, or urine), an organ, a tissue, a cell, asectional portion of an organ or tissue, a cell isolated from abiological subject (e.g., a region containing diseased cells, orcirculating tumor cells), a forensic sample or an ancient sample. Thebiological sample that contains, or is suspected to contain, the targetDNA may be of eukaryotic origin, prokaryotic origin, viral origin orbacteriophage origin. For example, the target DNA may be obtained froman insect, a protozoa, a bird, a fish, a reptile, a mammal (e.g., rat,mouse, cow, dog, guinea pig, or rabbit), or a primate (e.g., chimpanzeeor human) The target DNA may also be a complementary DNA (cDNA) that isgenerated from an RNA template (e.g., mRNA, ribosomal RNA) using areverse transcriptase enzyme. A DNA product generated by anotherreaction, such as a ligation reaction, a PCR reaction, or a syntheticDNA may also serve as a suitable target DNA. The target DNA may bedispersed in solution or may be immobilized on a solid support, such asin blots, arrays, glass slides, microtiter plates, beads or ELISAplates.

As used herein the term “oligonucleotide” refers to an oligomer ofnucleotides. A nucleotide may be represented by its letter designationusing alphabetical letters corresponding to its nucleoside. For example,A denotes adenosine, C denotes cytidine, G denotes guanosine, U denotesuridine, and T denotes thymidine (5-methyl uridine). W denotes either Aor T/U, and S denotes either G or C. N represents a random nucleosideand may be any of A, C, G, or T/U. A star (*) sign preceding a letterdesignation denotes that the nucleotide designated by the letter is aphosphorothioate-modified nucleotide. For example, *N represents aphosphorothioate-modified random nucleotide. A plus (+) sign preceding aletter designation denotes that the nucleotide designated by the letteris a locked nucleic acid (LNA) nucleotide. For example, +A represents anadenosine LNA nucleotide, and +N represents a locked random nucleotide.The oligonucleotide may be a DNA oligonucleotide, an RNA oligonucleotideor a DNA-RNA chimeric sequence. Whenever an oligonucleotide isrepresented by a sequence of letters, the nucleotides are in 5′→3′ orderfrom left to right. For example, an oligonucleotide represented by aletter sequence (W)_(x)(N)_(y)(S)_(z), wherein x=2, y=3 and z=1,represents an oligonucleotide sequence WWNNNS, wherein W is the 5′terminal nucleotide and S is the 3′ terminal nucleotide (“Terminalnucleotide” refers to a nucleotide that is located at a terminalposition of an oligonucleotide sequence. The terminal nucleotide that islocated at a 3′ terminal position is referred as a 3′ terminalnucleotide, and the terminal nucleotide that is located at a 5′ terminalposition is referred as a 5′ terminal nucleotide).

As used herein the term “nucleotide analogue” refers to compounds thatare structurally analogous to naturally occurring nucleotides. Thenucleotide analogue may have an altered phosphate backbone, sugarmoiety, nucleobase, or combinations thereof. Nucleotide analogues may bea synthetic nucleotide, a modified nucleotide, or a surrogatereplacement moiety (e.g., inosine). Generally, nucleotide analogues withaltered nucleobases confer, among other things, different base pairingand base stacking properties.

As used herein, the term “primer” or “primer sequence” refers to alinear oligonucleotide that hybridizes to a target DNA template togenerate a target DNA:primer hybrid and to prime a DNA synthesisreaction. Both the upper and lower limits of the length of the primerare empirically determined The lower limit on primer length is theminimum length that is required to form a stable duplex uponhybridization with the target nucleic acid under nucleic acidamplification reaction conditions. Very short primers (usually less than3 nucleotides long) do not form thermodynamically stable duplexes withtarget nucleic acid under such hybridization conditions. The upper limitis often determined by the possibility of having a duplex formation in aregion other than the pre-determined nucleic acid sequence in the targetnucleic acid. Generally, suitable primer lengths are in the range ofabout 3 nucleotides long to about 40 nucleotides long. The primer may bean RNA oligonucleotide, a DNA oligonucleotide, or a chimeric sequence.

As used herein, the term “random oligonucleotide” refers to a mixture ofoligonucleotide sequences, generated by randomizing a nucleotide at anygiven location in an oligonucleotide sequence in such a way that thegiven location may consist of any of the possible nucleotides or theiranalogues (complete randomization). A random oligonucleotide when usedas a random primer represents a random mixture of oligonucleotidesequences, consisting of every possible combination of nucleotideswithin the sequence. For example, a hexamer random primer may berepresented by a sequence NNNNNN or (N)₆. A hexamer random DNA primerconsists of every possible hexamer combinations of 4 DNA nucleotides, A,C, G and T, resulting in a random mixture comprising 4⁶ (4,096) uniquehexamer DNA oligonucleotide sequences. Random primers may be effectivelyused to prime a nucleic acid synthesis reaction when the target nucleicacid's sequence is unknown or for whole-genome amplification reaction.

As described herein, “partially constrained oligonucleotide” refers to amixture of oligonucleotide sequences, generated by completelyrandomizing some of the nucleotides of an oligonucleotide sequence(e.g., the nucleotide may be any of A, T/U, C, G, or their analogues)while restricting the complete randomization of some other nucleotides(e.g., the randomization of nucleotides at certain locations are to alesser extent than the possible combinations A, T/U, C, G, or theiranalogues). A partially constrained oligonucleotide may be used asprimer sequence. For example, a partially constrained DNA hexamer primerrepresented by WNNNNN, represents a mixture of primer sequences whereinthe 5′ terminal nucleotide of all the sequences in the mixture is eitherA or T. Here, the 5′ terminal nucleotide is constrained to two possiblecombinations (A or T) in contrast to the maximum four possiblecombinations (A, T, G or C) of a completely random DNA primer (NNNNNN).Suitable primer lengths of a partially constrained primer may be in therange of about 3 nucleotides long to about 15 nucleotides long.

As used herein the dNTP mixture refers to a mixture deoxyribonucleosidetriphosphates, where N is a random nucleotide including any of A, C, G,or T/U.

As used herein, the terms “strand displacing nucleic acid polymerase” or“a polymerase having strand displacement activity” refer to a nucleicacid polymerase that has a strand displacement activity apart from itsnucleic acid synthesis activity. A strand displacing nucleic acidpolymerase can continue nucleic acid synthesis on the basis of thesequence of a nucleic acid template strand by reading the templatestrand while displacing a complementary strand that is annealed to thetemplate strand.

As used herein, multiple displacement amplification (MDA) refers to anucleic acid amplification method, wherein the amplification involvesthe steps of annealing a primer to a denatured nucleic acid followed byDNA synthesis in which downstream double stranded DNA region(s) whichwould block continued synthesis is disrupted by a strand displacementnucleic acid synthesis through these regions. As nucleic acid issynthesized by strand displacement, single stranded DNA is generated bythe strand displacement, and as a result, a gradually increasing numberof priming events occur, forming a network of hyper-branched nucleicacid structures. MDA is highly useful for whole-genome amplification forgenerating high-molecular weight DNA from a small amount of genomic DNAsample with limited sequence bias. Any strand displacing nucleic acidpolymerase that has a strand displacement activity apart from itsnucleic acid synthesis activity (e.g., Phi29 DNA polymerase or a largefragment of the Bst DNA polymerase) may be used in MDA. MDA is oftenperformed under isothermal reaction conditions, using random primers forachieving amplification with limited sequence bias.

As used herein, the term “rolling circle amplification (RCA)” refers toa nucleic acid amplification reaction that amplifies a circular nucleicacid template (e.g., single stranded DNA circles) via a rolling circlemechanism RCA is initiated by the hybridization of a primer to acircular, often single-stranded, nucleic acid template. The nucleic acidpolymerase then extends the primer that is hybridized to the circularnucleic acid template by continuously progressing around the circularnucleic acid template to replicate the sequence of the nucleic acidtemplate over and over again (rolling circle mechanism). RCA typicallyproduces concatamers comprising tandem repeat units of the circularnucleic acid template sequence complement.

As used herein, the term “molecular crowding reagent” refers to thereagents or molecules, which alters the properties of other molecules ina solution. Examples of molecular crowding reagents include, but are notlimited to, dextran or polyethylene glycol (PEG). Generally, themolecular crowding reagents have high molecular weight, or bulkystructure which generates a crowded environment in a solution comprisingother molecules. The molecular crowding reagents reduce the volume ofsolvent available for other molecules in the solution, which results inmolecular crowding. In some embodiments, a high concentration ofpolyethylene glycol having a molecular weight of 6000 Da (PEG 6000)occupies a large proportion of the volume of a solution comprising othermolecules. For example, PEG 6000 present in a reaction mixturecomprising other reactants, wherein the PEG molecules occupy a largeproportion of the solvent of the reaction mixture. The molecularcrowding may alter the rates or equilibrium constants of the reactions.In some embodiments, wherein the melting temperature of theprimer-template DNA duplex (T_(m)) decreases in presence of low saltconcentration, that melting temperature is increased on addition of themolecular crowding reagents to the amplification reaction mixture.

As used herein, the term “reaction temperature” refers to a temperaturethat maintains during the amplification reaction. The embodiments of thepresent invention comprise an isothermal amplification reaction, whereinthe temperature of the reaction is constant. The entire isothermalamplification reaction is effected under the reaction temperature, suchas the reaction temperature for the isothermal amplification reactionusing GenomiPhi is about 30° C. The reaction temperature varies withvarying conditions, including but not limited to, use of differentpolymerases, size of the primer or template, use of additional salts orstabilizing agents.

As used herein, the term “melting temperature” (T_(m)) refers to atemperature at which one-half of a primer-template nucleic acid duplexdissociates generating single stranded oligomers, or nucleic acids suchas DNA. The stability of a primer-template DNA duplex may be measured byits T_(m). Primer length and sequence are of significant in designingthe parameters of a successful amplification. The melting temperature ofnucleic acid duplex increases with its length and with increasing GCcontent. The concentration of Mg²⁺, K⁺ and solvents influence the T_(m)of a primer. In one example, T_(m) is in a range of 15.8 to 27.8° C. inpresence of 85 mM K⁺/Na⁺ and GenomiPhi V2™, wherein the primer sequenceis NNNNN*N. In another example, T_(m) is in a range of 5 to 17° C. forsingle cell GenomiPhi™ in presence of 19 mM K⁺/Na⁺, wherein the primersequence is NNNNN*N.

The methods and kits described herein are intended to efficientlyamplify target nucleic acids with the additional advantage of reducingnon-specific amplification of non-target nucleic acids (e.g.,primer-dimers, chimeric nucleic acid products, etc.) that are observedwith other methods of nucleic acid amplification. Without intending tobe limited to a particular mechanism of action, the disclosed methodsaccomplish these goals by employing a molecular crowding reagent, a lowsalt condition and amplifying the nucleic acids under isothermalconditions.

One or more embodiments of a method comprise providing a target nucleicacid template, contacting the target nucleic acid template with areaction mixture; and amplifying the target nucleic acid template underisothermal amplification condition at a constant reaction temperature.In these embodiments, the reaction mixture comprises a DNA polymerasehaving a strand displacement activity, a deoxyribonucleosidetriphosphate (dNTP) mixture, a primer with a 3′ end and a 5′ end, amolecular crowding reagent, and a buffer solution, wherein the buffersolution maintains a salt concentration of the reaction mixture between10 to 30 mM and wherein the salt concentration of the reaction mixtureresults in melting temperature (T_(m)) of the primer to at least 10° C.below the reaction temperature. As noted, the reaction temperaturerefers to a single temperature that is maintained constant during theisothermal amplification reaction.

As noted, the salt concentration of the reaction mixture affects meltingtemperature (T_(m)) of the primer-target nucleic acid duplex, whereinthe resulting melting temperature is less than the reaction temperature.In one or more embodiments, the melting temperature of the primer-targetnucleic acid duplex is decreased in presence of low salt condition in anamplification reaction. In one or more embodiments, the duplex meltingtemperature (T_(m)) of the oligonucleotide primer(s) is (are) 8-10° C.lower under low salt condition, as the stability of the Watson-Crickbase pairing in the nucleic acid-primer hybrid may decrease under thatcondition. In one or more examples, the reaction rate increases inpresence of PEG in the amplification reaction mixture under low saltconcentration compared to the reaction rate of the same reaction inabsence of PEG. The experimental observation established the fact thatthe amplification reaction had very slow reaction kinetics withdecreasing salt concentration, such as between 10 to 15 mM. The reactionrate of the amplification reaction improves on addition of molecularcrowding reagents, such as PEG, under the same low salt conditions. Themelting temperature of the primers (oligomers) decreases at low saltconcentration, which also decreases the reaction kinetics, wherein thekinetics of the amplification reaction further increases by addingmolecular crowding reagents. The higher reaction temperature than theT_(m) of a duplex may cause destabilization of the primer-templateduplex and may melt the duplex, as the melting temperature of theprimers are low under this low salt condition. However, unexpectedly, onaddition of molecular crowding reagents, such as PEG, there is observedan increase in the reaction rate and the amplification reaction proceedsas other conventional amplification reaction, and therefore theprimer-template duplex stabilizes at relatively higher temperature. Forexample, even when the melting temperature of the primer-template duplexis 15° C., the duplex is stabilized at 30° C. in presence of molecularcrowding reagents under the low salt condition. The melting temperatureof the same primer-template duplex decreases under low salt condition,and it may be decreased by 8-10° C. In some embodiments, the decreasedT_(m) of the duplex requires the amplification reaction to be performedat a lower temperature than that used in traditional amplificationreactions.

As noted, the method comprises contacting the target nucleic acidtemplate with a reaction mixture comprising a molecular crowdingreagent. The molecular crowding reagent serves to increase the speed andefficiency of amplification reaction under stringent condition. Themolecular crowding reagents may increase the rate of the reaction. Insome embodiments, the low salt concentration optimizes the T_(m) suchthat the T_(m) is at least 10° C. lower than the reaction temperature,and the presence of molecular crowding reagents results desirednon-biased amplification products with increased reaction rate.

The melting temperature of a primer-template duplex may be calculatedusing various standard methods. The melting temperature of the presentmethod is calculated using the method described below. The calculationsare only estimates the melting temperature and different factors mayaffect the melting temperature, including detergents, saltconcentrations, counter ions or solvents. As the T_(m) is modified byusing a low salt condition, in some embodiments, the T_(m) may refer toherein as a salt adjusted melting temperature (T_(m)). For thedetermination of Tm, a variation on two standard approximationcalculations is used. For sequences less than 14 nucleotides the sameformula as the basic calculation is use, with a salt concentrationadjustment:

T _(m)=(wA+xT)*2+(yG+zC)*4−16.6*log₁₀(0.050)+16.6*log₁₀([Na⁺)

wherein, w, x, y, and z are the number of bases A, T, G, C in theoligonucleotides sequence, respectively. The term 16.6*log₁₀ ([Na⁺])adjusts the T_(m) for changes in the salt concentration, and the termlog₁₀(0.050) adjusts for the salt adjustment at 50 mM Na⁺. The reactionmixture may contain one or more monovalent and divalent salts which mayhave an effect on the T_(m) of the oligonucleotides. As the sodium ionsare much more effective at forming salt bridges between DNA strands andtherefore have significant effect in stabilizing double-stranded DNA.The melting temperature (Tm) calculation assumes that the annealingoccurs under the standard condition of 50 mM primer at pH7.0 in presenceof monovalent cation (either Na+ or K+) with concentrations between 0.01and 1.0 M, the non-symmetric sequences are at least 8 bases long andcontain at least one G or C. (See Nakano et al, (1999) Proc. NucleicAcids Res. 27:2957-65, and Warren A Kibbe, (2007) Nucleic Acids Res. 35:W43-W46 (webserver issue)).

The kinetics of amplification using MDA is increased by incorporation ofmolecular crowding reagents in the reaction mixture. The molecularcrowding is a factor that determines the structure, stability andfunction of nucleic acids. In some embodiments, the structure andstability of the DNA duplexes are influenced by the molecular crowdingreagents. The molecular crowding reagents may affect the nucleic acidstructures, which may depend on the patterns of base-pairing or hydrogenbonding in the nucleic acid structure. Different size of the molecularcrowding reagents has different effect on stabilization of the DNAduplexes, wherein the length of the DNA duplex also contributes to thestability. For example, polyethylene glycol (PEG) is a molecularcrowding reagent which has varying molecular weight, and has differenteffect on the stability of the DNA duplex. In some embodiments, theamplification reaction rate increases on addition of PEG in presence oflow salt condition.

In one or more embodiments, the molecular crowding reagent used in theamplification reaction is selected from a group consisting of apolyethylene glycol, Ficoll™, trehalose and combinations thereof. In oneembodiment, the molecular crowding reagent comprises polyethyleneglycol. The molecular crowding reagent may be selected from a groupconsisting of a PEG 2000, PEG 6000, PEG 8000 and combinations thereof.In some embodiments, the amplification reaction employs 2.5% PEG-8000that increases the amplification rate compared to the standardamplification conditions, such as GenomiPhi™ condition.

In one or more embodiments of the nucleic acid amplification reactions,a high stringency hybridization condition may be employed to reduceundesired amplification products and artifacts. High stringencyhybridization conditions refer to reaction conditions that impart ahigher stringency to an oligonucleotide hybridization event than thestringency provided by conditions that are generally used for nucleicacid amplification reactions. Typically, nucleic acid amplificationreaction is performed wherein the T_(m) of the oligonucleotide primer(s)is/are within 10 degrees of the reaction temperature used foramplification. This allows the oligonucleotide primer(s) to bind stablyto the template. Under high stringency conditions, the T_(m) of theoligonucleotide primer(s) used is greater than the temperature that is10 degrees lower than the reaction temperature. This may prevent stablebinding of the primer to the template, and is not typically used foramplification reactions. For example, a high stringency hybridizationcondition may be achieved in a nucleic acid amplification reaction byincreasing the reaction temperature or by decreasing the saltconcentration. A combination of low salt (˜15 mM) and the use of amolecular crowding reagent (e.g., 2.5% PEG-8000) provided increasedreaction kinetics and more uniform coverage of amplified sequences, asshown in FIGS. 1A, 1B, 1C, 2 and 3.

As noted, the method comprises a step of contacting the target nucleicacid template with a reaction mixture comprising a buffer solution. Thebuffer solution used for amplification reaction may have 1 to 75 mM saltconcentration. In some embodiments, the salt concentration is between 1to 35 mM. In some embodiments, the buffer solution maintains a saltconcentration of the reaction mixture between 10 to 30 mM. In oneembodiment, the salt concentration of the reaction mixture is maintainedat about 20 mM. In some other embodiments, the amplification reactionoccurs under a lower concentration of salt compared to the conventionalamplification methods. For example, 15 mM KCl is used for amplificationas opposed to the 75 mM KCl used in traditional amplification reactions.Nucleic acid amplification reactions that utilize random hexamers areoften carried out at about 75 mM salt concentration and at 30° C.,wherein the embodiments of the method comprise the step of nucleic acidamplification reaction at about 15 mM salt concentration and 30° C.,which is a high stringency hybridization condition. The amplificationreaction under low salt concentration in the presence of molecularcrowding agents improves the speed and sensitivity of the reaction whenamplifying from trace nucleic acid samples.

In some embodiments, the duplex melting temperature (T_(m)) is decreasedby about 5-10° C. under low salt concentration, but molecular crowdingagents are added which allows the amplification reaction to be performedunder more stringent conditions, such as at a higher temperature. Saltconcentration may be varied depending on length of the primer as well asconstituents of the nucleotides of the primer to result decreasedmelting temperature. The salt used for the present method may bemonovalent salt, such as sodium or potassium.

In embodiments of the present method, the resulting melting temperatureof the primer-template duplex at 75 mM salt concentration is in a rangeof 15-27° C., wherein the reaction is performed at 30° C. in presence ofrandom hexamer primer and in the absence of molecular crowding agents.When the reaction condition is modified to 15 mM salt concentration at30° C. reaction temperature, in presence of random hexamer primer and inthe absence of molecular crowding agents, the resulting meltingtemperature of the primer-template duplex is in a range of 3-15° C.,which is more than 15° C. lower than the reaction temperature. Underthis 15 mM salt concentration condition, the reaction kinetics areexpected to slow considerably. However, unexpectedly, the reaction rateincreases in presence of PEG under 15 mM salt concentration, 30° C.reaction temperature, in presence of random hexamer primer compared tothe reaction rate in absence of PEG under the same condition, which maybe caused by better primer-template hybridization occurs in presence ofPEG. In addition to better reaction kinetics, molecular crowdingreagents (PEG) also result in producing better representativeamplification product.

As noted, in some embodiments, an isothermal amplification methodcomprises the steps of providing a target DNA. Sufficient quantity of atarget nucleic acid is one of the primary requirements for anamplification reaction to generate evenly amplified nucleic acid withcorrect sequence. Under standard GenomiPhi™ reaction conditions, thetarget nucleic acid of quantity greater than ˜1-10 ng generatesnon-biased amplification products, whereas more biased amplificationoccurs throughout the genome when the input target nucleic acid quantityis less. Using less quantity of target nucleic acids, certain areas ofthe genome are amplified with reduced efficiency, wherein some of theareas of the genome are amplified with increased efficiency, resultingin non-uniform amplification product. The dropouts and high levels ofamplification bias or missing sequences in the amplified products arereduced in spite of using a low quantity of input target nucleic acids,such as femtogram levels of input target nucleic acids in the presentembodiments of the methods. One or more embodiments of the methodreduces the probability of forming defective amplicons especially whenattempting to amplify DNA from a single cell which contains a limitedquantity of nucleic acid to start an amplification reaction. Forexample, human cells contain ˜6.6 pg of DNA, wherein bacterial cellscontain ˜5 fg of DNA. Even after using a target nucleic acid of about 5fg, which may be available from a single microbial cell, the presentmethod of amplifying nucleic acids results in amplicons with low levelsof incorrect sequences or amplification bias. In one or moreembodiments, at least about 5 fg target nucleic acid template isprovided, or essentially one microbial genome. In some examples, theamplification from a single bacterial cell, wherein at least about 5 fgof bacterial target nucleic acid is available, results in higher levelsof target sequence and more representative amplification product underthe condition of low salt and presence of molecular crowding reagentcompared to the standard amplification method. In some otherembodiments, the amplification from a single human cell, wherein atleast about 6.6 pg of the human target nucleic acid is available, whichresults in more complete and representative amplification product underthe same condition of low salt and molecular crowding reagent. In theseembodiments, the amplification rates are also increased compared to thestandard amplification conditions.

The target DNA may be linear template, nicked template or a circulartemplate. It may be a natural or synthetic DNA. The target DNA may be acDNA or a genomic DNA. The DNA template may be a synthetic template(e.g., a linear or nicked DNA circularized by enzymatic/chemicalreactions), or it may be a plasmid DNA.

As noted, the primer used for the amplification method has a low meltingtemperature. For priming DNA synthesis, the amplification reaction oftenutilizes random hexamers with the sequence 5′-NNNN*N*N, where “N”represents a deoxyadenosine (dA), deoxycytidine (dC), deoxyguanosine(dG), or deoxythymidine (dT) and “*” represents a phosphorothioatelinkage.

In one or more embodiments, the primer may be a specific primersequence, a random primer sequence or a partially constrained randomprimer sequence. Specific primer sequences are complementary to aparticular sequence that is present in the target DNA template, in theWatson-Crick base-pair. Specific oligonucleotide sequences may beemployed in the primer, for example, for specifically amplifying amitochondiral DNA in a mixture, a certain plasmid in a mixture, orcertain genome region.

In some embodiments, the oligonucleotide sequence in the primer is arandom primer sequence. In some embodiments, the length of the primersis between 5 to 9 nucleotides long. In one embodiment, the primer lengthis 6 nucleotides (hexamer). For example, the primer may be a randomhexamer sequence. In embodiments of amplification reaction using randomhexamer primer under the condition of 15 mM salt, 30° C. reactiontemperature, the melting temperature of the primer-template duplexdecreases to at least 10° C. below the reaction temperature.

Further, the random sequence may comprise one or more modifiednucleotides and may comprise one or more phosphorothioate linkages. Forexample, the primer may be NN(N)mNN, where the integer value of m rangesfrom 0 to 36. In some embodiments, the integer value of m may range from0 to 20. In some other embodiments, the integer value of m may rangefrom 0 to 10. In some example embodiments, the oligonucleotide sequencemay be a random tetramer, a random pentamer, a random hexamer, a randomheptamer or a random octamer. The primer may comprise natural, syntheticor modified nucleotides, or nucleotide analogues. For priming DNAsynthesis, the amplification reaction frequently utilizes randomhexamers with the sequence 5′-NNNNN*N, where “N” represents adeoxyadenosine (dA), deoxycytidine (dC), deoxyguanosine (dG), ordeoxythymidine (dT) and “*” represents a phosphorothioate linkage. Insome embodiments, the primers are modified to minimize competingnon-target nucleic acid (i.e., template DNA) amplification to modify theoligonucleotide primers in such a way as to inhibit their ability toanneal with one another.

Constrained-randomized hexamer primers that cannot cross-hybridize viaintra- or inter-molecular hybridization (e.g., R6, where R=A/G) havebeen used for suppressing primer-dimer structure formation duringnucleic acid amplification. These constrained-randomized primers,however, impart considerable bias in nucleic acid amplificationreaction. Such primers are also of limited use for sequence-non-specificor sequence-non-biased nucleic acid amplification reactions (e.g., wholegenome or unknown nucleic acid sequence amplification).

In some embodiments, the primer may comprise synthetic backbones ornucleotide analogues that confer stability and/or other advantages(e.g., secondary structure formation) to the primers (e.g., peptidenucleic acid or PNA), locked nucleic acid (LNA) or may comprise modifiedsugar moieties (e.g., xylose nucleic acid or analogues thereof).

In some embodiments, the primer comprises one or more LNA nucleotides.The speed and sensitivity of the amplification reaction, such as MDA maybe improved when amplifying from trace nucleic acid samples using LNAsinto the oligonucleotide primers. LNAs are a class of conformationallyrestricted nucleotide analogues that serve to increase the speed,efficiency, and stability of base pairing, thereby promoting thehybridization of the modified oligonucleotides to their target sequencesin the nucleic acid of interest. LNA nucleotide contains a bicyclicfuranose sugar unit locked in a ribonucleic acid-mimicking sugarconformation. The structural change from a deoxyribonucleotide (or aribonucleotide) to the LNA nucleotide may be limited from a chemicalperspective, for example, the introduction of an additional linkagebetween carbon atoms at 2′ position and 4′ position (e.g., 2′-C,4′-C-oxymethylene linkage). The 2′ and 4′ position of the furanose unitin the LNA nucleotide may be linked by an O-methylene (e.g., oxy-LNA:2′-O, 4′-C-methylene-β-D-ribofuranosyl nucleotide), a S-methylene(thio-LNA), or a NH-methylene moiety (amino-LNA), and the like. Suchlinkages restrict the conformational freedom of the furanose ring. Insome embodiments, the primers comprising one or more LNAoligonucleotides display enhanced hybridization affinity towardscomplementary single-stranded RNA, single-stranded DNA ordouble-stranded DNA. Further, inclusion of LNA in the oligonucleotidemay induce A-type (RNA-like) duplex conformations.

In some embodiments, the nucleic acid amplification uses random hexamerprimers of the general structure 5′-+W+WNNN*S-3′, where “+” precedes alocked nucleic acid base (i.e., “an LNA base”; for example, +A=anadenosine LNA molecule), “W” represents a mixture of only dA and dT, and“S” represents a mixture of only dC and dG. The “*” represents aphosphorothioate linkage between the two nucleotides. Since “W” basesare unable to stably pair with “S” bases, the formation of theoligonucleotide duplex is inhibited, which leads to decreasedamplification of non-template nucleic acids.

In some embodiments, the primer employed for DNA amplification reactionmay be resistant to nucleases, for example an exonuclease. For example,the primer may comprise one or more modified phosphate linkage (e.g., aphosphorothioate linkage) to render it exonuclease-resistant. In someembodiments, the primer comprises an exonuclease-resistant randomoligonucleotide sequence. For example, the primer may have a randomsequence such as NNNNN*N or NNNN*N*N.

In some embodiments, the primer is a partially constrained primersequence. Non-limiting examples of partially constrained primersequences, that have restricted randomization only at the terminalnucleotides include, but is not limited to, W(N)_(y)S, S(N)_(y)W,D(N)_(y)G, G(N)_(y)D, C(N)_(y)A, or A(N)_(y)C. The integer value of ymay be in the range 2 to 13. In some embodiments, the value of y may be2, 3, 4, or 5. In some example embodiments, a partially constrainedprimer sequence, (W)_(x)(N)_(y)(S)_(z), wherein x, y and z are integervalues independent of each other, and wherein value of x is 2 or 3,value of y is 2, 3, 4, or 5 and value of z is 1 or 2. The partiallyconstrained primer sequence may comprise one or more nucleotideanalogues. In some embodiments, the partially constrained primersequence may have a terminally mismatched primer-dimer structure. Forexample, since W cannot base pair with S, there will be a terminalmismatch at both the 3′ terminal nucleotides if the primer-dimerstructure without any recessed ends is formed by inter-molecularhybridization. In some embodiments, the primer sequence is anuclease-resistant, partially constrained sequence comprising a modifiednucleotide, and having terminal mismatch primer-dimer structure.

In some embodiments, methods for producing at least one amplicon basedon a target DNA comprise the steps of providing the target DNA,annealing at least a primer to the target DNA to generate a targetDNA:primer hybrid, and extending the primer via an isothermal nucleicacid amplification reaction to produce at least one amplicon that iscomplementary to at least one portion of the target DNA.

The nucleic acid polymerase used for the isothermal amplificationmethods may be a proofreading or a non-proofreading nucleic acidpolymerase. The nucleic acid polymerase may be a thermophilic or amesophilic nucleic acid polymerase. Examples of DNA polymerases that aresuitable for use in the methods include, but are not limited to, Phi29DNA polymerase, hi-fidelity fusion DNA polymerase (e.g., Pyrococcus-likeenzyme with a processivity-enhancing domain, New England Biolabs, MA),Pfu DNA polymerase from Pyrococcus furiosus (Strategene, Lajolla,Calif.), Bst DNA polymerase from Bacillus stearothermophilus (NewEngland Biolabs, MA), Sequenase™ variant of T7 DNA polymerase, exo(−)Vent_(R)™ DNA polymerase (New England Biolabs, MA), Klenow fragment fromDNA polymerase I of E. coli, T7 DNA polymerase, T4 DNA polymerase, DNApolymerase from Pyrococcus species GB-D (New England Biolabs, MA), orDNA polymerase from Thermococcus litoralis (New England Biolabs, MA).

In some embodiments, the nucleic acid polymerase used for the isothermalamplification is a strand displacing nucleic acid polymerase. Themethods may employ a highly processive, strand-displacing polymerase toamplify the target DNA under conditions for high fidelity baseincorporation. A high fidelity DNA polymerase refers to a DNA polymerasethat, under suitable conditions, has an error incorporation rate equalto or lower than those associated with commonly used thermostable PCRpolymerases such as Vent DNA polymerase or T7 DNA polymerase (from about1.5×10⁻⁵ to about 5.7×10⁻⁵). In some embodiments, a Phi29 DNA polymeraseor Phi29-like polymerase may be used for amplifying a DNA template. Insome embodiments, a combination of a Phi29 DNA polymerase and a Taq DNApolymerase may be used for the circular DNA amplification.

Additional enzymes may be included in the isothermal amplificationreaction mixture to minimize mis-incorporation events. For example,protein-mediated error correction enzymes, such as, MutS, may be addedto improve the DNA polymerase fidelity either during or following theDNA polymerase reaction.

In some embodiments, one or more amplicons are produced from a circulartarget DNA template by rolling circle amplification (RCA). Theamplification reagents including a DNA polymerase, primer, dNTPs andmolecular crowding reagents and a buffer to maintain low saltconcentration may be added to the target DNA to produce an amplificationreaction mixture for initiating an RCA reaction. The amplificationreaction mixture may further include reagents such as single-strandedDNA binding proteins and/or suitable amplification reaction buffers.After or during the amplification reaction, amplicons may be detected byany of the currently known methods for DNA detection. RCA may be alinear RCA (LRCA), exhibiting linear amplification kinetics (e.g., RCAusing a single specific primer), or may be an exponential RCA (ERCA)exhibiting exponential amplification kinetics. RCA may also be performedusing multiple primers (multiply primed rolling circle amplification orMPRCA) leading to hyper-branched concatemers. For example, in adouble-primed RCA, one primer may be complementary, as in the linearRCA, to the circular nucleic acid template, whereas the other may becomplementary to the tandem repeat unit nucleic acid sequences of theRCA product. Consequently, the double-primed RCA may proceed as a chainreaction with exponential (geometric) amplification kinetics featuring aramifying cascade of multiple-hybridization, primer-extension, andstrand-displacement events involving both the primers. This oftengenerates a discrete set of concatemeric, double-stranded nucleic acidamplification products. In some example embodiments, an RCA is performedin vitro under isothermal conditions using a suitable nucleic acidpolymerase such as Phi29 DNA polymerase.

In some other embodiments, a linear DNA template may be amplified usingMDA. Conventional methods of MDA using random primers and 75 mM salt at30° C. can result in sequence-biased amplification and the formation ofchimeric products. When the salt concentration is lowered in thesereactions to 15 mM salt in an attempt to reduce production of chimericsequences, the reaction kinetics slowed considerably. In contrast, usageof molecular crowding reagents and low salt condition in MDA reactionpromoted faster DNA amplification kinetics and improved DNA sequencecoverage and balance. Further, the decrease in T_(m) of the targetDNA:primer hybrid allows the MDA reaction to be performed under morestringent conditions, such as at a lower concentration of salt (e.g., 15mM KCl as opposed to 75 mM under otherwise standard conditions) orallows use of more stringent buffers for high stringent hybridizationconditions. Such stringent reaction further decreases unwanted reactionintermediates and products such as formation of chimeric products byself-hybridization.

Further, usage of molecular crowding reagent and low salt concentrationin amplification reactions allows for robust amplification of trace DNAsamples under a wider variety of conditions, including but not limitedto, circulating plasma DNA, DNA isolated from formalin fixedparaffin-embedded (FFPE) samples, forensics DNA samples that have beenexposed to environmental conditions or ancient DNA samples. Theamplified library comprising the amplicons may further be used fortargeted detection of amplified sequences via qPCR or sequencing.

In some embodiments, a kit for isothermal DNA amplification is provided.The kit comprises a DNA polymerase having strand displacement activityand a primer, a molecular crowding reagent, a buffer solution, whereinthe buffer solution maintains a salt concentration of 10 to 30 mM.

In some embodiments, the kit comprises a Phi29 DNA polymerase. The kitmay further comprise one or more random primers which have lower meltingtemperature.

The methods and kits described herein may be used for amplifying andanalyzing DNA samples such as those for forensic analysis, bio-threatidentification, or medical analysis. The sensitivity of the methodallows for the whole-genome amplification of single bacterial andeukaryotic cells for whole genome amplification for downstream testingand analysis. Further, the use of molecular crowding reagents and lowsalt condition promote faster DNA amplification kinetics, highersensitivity for low input DNA quantities, and results in less biased,more balanced amplification.

The following examples are disclosed herein for illustration only andshould not be construed as limiting the scope of the invention. Someabbreviations used in the examples section are expanded as follows:“mg”: milligrams; “ng”: nanograms; “pg”: picograms; “fg”: femtograms;“mL”: milliliters; “mg/mL”: milligrams per milliliter; “mM”: millimolar;“mmol”: millimoles; “pM”: picomolar; “pmol”: picomoles; “μL”:microliters; “min.”: minutes and “h.”: hours.

EXAMPLES Example 1 Reaction Kinetics and Sensitivity of MDA Reactions ofDNA from Single Cells in Presence of PEG and Low Salt Condition

Cultures of E. coli MG1655 were grown to log phase in LB media,harvested by centrifugation, and washed three times using TEN buffer (10mM Tris, pH 7.5, 100 mM NaCl, and 0.1 mM EDTA). After washing, cellswere resuspended in buffer TEN+30% glycerol and serial dilutions weremade. Cells were then stained with 10 μM FM1-43FX dye (F-35355, fromInvitrogen, Life Technologies) for 10 minutes at room temperature, addedstained cells into each of the wells of a transparent-bottom 384-wellplate, and counted using an inverted fluorescent microscope (NikonEclipse TE2000-U). Following identification of wells containing singlecells, lysis was initiated by addition of 2 μl of 0.2 M KOH, 50 mM DTT,0.015% Tween-20 and freezing at −80° C. overnight. The followingmorning, plates were thawed and lysate was further incubated at 65° C.for 10 minutes, cooled, and neutralized by addition of 1 μl of 0.4M HCl,0.6 M Tris, pH 7.5.

Amplification (GenomiPhi™) reactions were performed using a randomhexamer primer in presence and absence of PEG 8000 and low saltcondition to determine the effect of molecular crowding reagent and lowsalt condition on MDA reactions. GenomiPhi™ amplification reactionmixtures containing 50 mM HEPES, pH 8.0, 20 mM MgC12, 0.01% Tween-20, 1mM TCEP, 40 μM random hexamer, SYBR Green I (Invitrogen) at 1:20,000dilution, 20 μg/ml Phi29 polymerase, and the indicated concentrations ofPEG-8000 and KCl (Table 1) were prepared by incubating at 30° C. for 1hour to remove any small quantity of contaminating DNA. Amplificationreactions were initiated by addition of 400 μM dNTPs to the celllysates. Reactions were incubated at 30° C. for 8 hours in a platereader (Tecan), while taking fluorescence measurements at 5 minintervals to measure amplification kinetics. The amplification reactionwas monitored real time by measuring the fluorescence increase over timein a Tecan plate reader (Tecan SNiPer, Amersham-Pharmacia Biotech).Reactions were then inactivated by heating at 65° C. for 20 minutes andamplified DNA was purified by ethanol precipitation. The average DNAyields as determined by quantitation by Pico Green (Invitrogen) areshown in table 1, wherein dN6 represents a hexamer primer having anoligonucleotide sequence of NNNN*N*N. The salt concentrations in thesereactions are listed assuming that 20 mM of salt originates from thecell lysis and neutralization procedure. The remaining salt comes fromadditional KCl added to the reaction mixture. The average yield is fromthree 20 μl reactions, except for the clean GenomiPhi™ formulation inwhich only two reactions produced amplified product.

TABLE 1 The average DNA yields as determined by Pico Green AssayFormulation Hexamer 2.5% PEG-8000 Salt (mM) Avg. yield (μg) Clean GPhidN6 N 75 1.8 dN6 − PEG dN6 N 20 3.28 dN6 + PEG dN6 Y 20 3.03

Fluorescence measurements were taken at 5 min intervals of amplificationreactions from 5 fg of purified E. coli DNA (approximately the amount ofDNA from a single cell), from reactions containing lysed single cell,and from reactions in which no DNA was added (NTC) as shown in FIGS. 1Ato 1C, wherein RFU, relative fluorescence units; was measured withrespect to time. NTC represents the “no template control” wherein theamplification reaction was performed without the addition of a targetDNA template. The reduction of salt in the dN6 −PEG formulation allowedfor all three single cells to be amplified and provided a higher averageyield of amplified DNA product. FIGS. 1A, 1B and 1C illustrate theamplification kinetics of target nucleic acids using standard randomhexamer primer in absence of PEG under high salt condition, in absenceof PEG under low salt condition, and in presence of PEG under low saltcondition, respectively. The amplification rate for no template control(NTC), 5 fg DNA, and each of the single cell samples were estimated bymonitoring the time taken for the generation of detectable levels ofamplicon products in each of the samples.

FIG. 1C shows an increased reaction kinetics and sensitivity of MDAreaction in presence of PEG under low salt condition. The amplificationreaction in presence of PEG under low salt condition provided increasedamplification speed (approximately 2.5-fold) and allowed femtogram (fg)quantities of DNA, and DNA from a single cell to be amplifiedefficiently. An analysis of the reaction kinetics (FIGS. 1A to 1C)showed that the amplification rates for the clean GenomiPhi™ formulationand the dN6 −PEG formulation were approximately the same. However, uponaddition of 2.5% PEG-8000 as in the dN6 +PEG formulation, theamplification kinetics was dramatically improved, displaying anapproximately 2.5-fold increase. In addition, there was a clearseparation in amplification time between the single cells as comparedwith the no-template control, suggesting a higher quality of amplifiedproduct.

The kinetics and yield, unexpectedly, suggest that in spite of providingreaction conditions of low salt and presence of molecular crowdingreagents, the amplification reaction containing PEG and low saltproceeded quickly and efficiently. Unlike the currently known methods,which disclose the condition of low salt prevents efficient primerhybridization, the present method showed in presence of both low saltand molecular crowding reagents, the reaction rate increases compared tothe same at low salt condition without molecular crowding reagents. Thereaction conditions which generally prevents efficient hybridization ofprimer-template, include but are not limited to, a low saltconcentration, a high temperature and use of primers having a T_(m)which is 8-10° C. lower than that of the reaction temperature.Additionally, analysis of the amplified product indicates that allregions of the template genomic DNA were amplified representatively,with no under-amplification of A/T rich regions, and over-amplificationof G/C rich regions, which would be typically expected under conditionswhere the primer binding conditions were stringent, and only allowed forG/C rich primers to hybridize. Moreover, the presence of PEG as amolecular crowding agent and low salt condition did not inhibit bindingof the primer and template DNA and extension by DNA polymerase, andstrand displacement of the primer by the DNA polymerase duringisothermal amplification. If PEG or low salt condition inhibited thehybridization of the primer-template, followed by initiation ofextension by the DNA polymerase, the kinetics would have been slow, andyield would have been low.

The amplified DNA from the single cell was processed into libraries andsubjected to whole-genome sequencing using the Illumina HiSeq™ 2000 withpaired-end reads and 100 base pair read lengths. Approximately 8 millionto 16 million reads were obtained for each sample, which were thenmapped to the E. coli MG1655 reference genome, as shown in FIG. 2. Rawreads were uploaded to DNANexus, which offers DNA analysis computingservices and the reads were mapped to the E. coli MG1655 referencegenome. Only reads that were mapped accurately were included in theanalysis including reads that mapped repetitively. The standarddeviation between the single cell replicates is indicated by the errorbars, as shown in FIG. 2.

From FIG. 2, a clear difference for three different GenomiPhi™amplification formulations in producing DNA reads that were mappedsuccessfully. The lower salt concentration and the presence of PEG-8000together allowed for the production of more target E. coli DNA andcorrespondingly less unmappable sequence.

After mapping reads to the E. coli MG1655 reference genome, thepercentage of bases in the total 4.6 Mb genome that were covered by atleast the indicated number of reads (1×-100×) was calculated, as shownin FIG. 3, wherein the standard deviation between the single cellreplicates is indicated by the error bars. In addition, the combinationof low salt and the addition of PEG-8000 allowed for a greater degree ofbase coverage of the E. coli genome by the sequence read which wasevident at a range of coverage depths from 1× to 100× (FIG. 3). Thiswould allow for an improved ability to construct genome sequences ofunknown sequence and for a more robust analysis in sequencingapplications.

The above detailed description is exemplary and not intended to limitthe invention of the application and uses of the invention. Throughoutthe specification, exemplification of specific terms should beconsidered as non-limiting examples. The singular forms “a”, “an” and“the” include plural referents unless the context clearly dictatesotherwise. Approximating language, as used herein throughout thespecification and claims, may be applied to modify any quantitativerepresentation that could permissibly vary without resulting in a changein the basic function to which it is related. Accordingly, a valuemodified by a term such as “about” is not to be limited to the precisevalue specified. Unless otherwise indicated, all numbers expressingquantities of ingredients, properties such as molecular weight, reactionconditions, so forth used in the specification and claims are to beunderstood as being modified in all instances by the term “about.”Accordingly, unless indicated to the contrary, the numerical parametersset forth in the following specification and attached claims areapproximations that may vary depending upon the desired propertiessought to be obtained by the invention. At the very least, and not as anattempt to limit the application of the doctrine of equivalents to thescope of the claims, each numerical parameter should at least beconstrued in light of the number of reported significant digits and byapplying ordinary rounding techniques. Where necessary, ranges have beensupplied, and those ranges are inclusive of all sub-ranges therebetween.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The foregoingembodiments are selected embodiments or examples from a manifold of allpossible embodiments or examples. The foregoing embodiments aretherefore to be considered in all respects as illustrative rather thanlimiting on the invention. While only certain features of the inventionhave been illustrated and described herein, it is to be understood thatone skilled in the art, given the benefit of this disclosure, will beable to identify, select, optimize or modify suitableconditions/parameters for using the methods in accordance with theprinciples of the invention, suitable for these and other types ofapplications. The precise use, choice of reagents, choice of variablessuch as concentration, volume, incubation time, incubation temperature,and the like may depend in large part on the particular application forwhich it is intended. It is, therefore, to be understood that theappended claims are intended to cover all modifications and changes thatfall within the spirit of the invention. Further, all changes that comewithin the meaning and range of equivalency of the claims are intendedto be embraced therein.

1. A kit for amplifying a nucleic acid comprising: (a) a DNA polymerase having a strand displacement activity; (b) a molecular crowding reagent; and (c) a buffer that provides a final salt concentration between 10 mM to 20 mM during amplification.
 2. A kit for isothermal amplification of a nucleic acid comprising: (a) a DNA polymerase having a strand displacement activity; (b) a molecular crowding reagent; and (c) a buffer that provides a salt concentration between 1 mM and 35 mM in an isothermal amplificaiton reaction mixture, wherein the molecular crowding reagent is a polyethylene glycol (PEG).
 3. The kit of claim 2, wherein the buffer provides a salt concentration between 10 mM and 30 mM in the isothermal amplificaiton reaction mixture.
 4. The kit of claim 2, wherein the buffer provides a salt concentration between 10 mM and 20 mM in the isothermal amplificaiton reaction mixture.
 5. The kit of claim 2, wherein the buffer provides a salt concentration of about 20 mM in the isothermal amplificaiton reaction mixture. 6 The kit of claim 2, wherein the PEG is selected from a group consisting of PEG-400, PEG-2000, PEG-6000, PEG-8000, and combinations thereof.
 7. The kit of claim 6, wherein the PEG is PEG-8000.
 8. The kit of claim 7, wherein a concentration of the PEG-8000 in the isothermal amplificaiton reaction mixture is about 2.5 wt %.
 9. The kit of claim 8, wherein the buffer provides a salt concentration of 20 mM in the isothermal amplificaiton reaction mixture.
 10. The kit of claim 2 further comprising a random primer.
 11. The kit of claim 10, wherein the random primer is a random hexamer primer.
 12. The kit of claim 11, wherein the random hexamer primer has a general structure of (atN)(atN)(atN)(atN)(atN)*N, wherein (atN) represents any of 2-amino-deoxyadenosine (2-amino-dA), dC, dG, or 2-thio-deoxythymidine (2-thio-dT).
 13. The kit of claim 11, wherein the random hexamer primer comprises at least one of a 2-amino-deoxyadenosine (2-amino-dA) or a 2-thio-deoxythymidine (2-thio-dT).
 14. The kit of claim 2, wherein the DNA polymerase is a phi29 DNA polymerase.
 15. The kit of claim 14 further comprising a random primer having a melting temperature of lower than 20° C. in the isothermal amplification reaction mixture.
 16. The kit of claim 2 further comprising deoxyribonucleoside triphosphate (dNTP) mixture.
 17. A kit for isothermal amplification of a nucleic acid comprising: (a) a phi29 DNA polymerase; (b) a molecular crowding reagent; (c) a buffer that provides a salt concentration of about 20 mM in an isothermal amplificaiton reaction mixture; and (d) a random hexamer primer having an oligonucleotide sequence NNNN*N*N, wherein the molecular crowding reagent is polyethylene glycol-8000 (PEG-8000).
 18. The kit of claim 17, wherein a melting temperature of the random hexamer primer in the isothermal amplification reaction mixture is lower than 20° C. 